In recent years, injectors, for fuel injection for automobiles, using laminate-type piezoelectric elements have been developed from the standpoint of improving fuel efficiency, exhaust gas emission, etc. of automobiles.
The laminate-type piezoelectric element has a ceramic laminated body obtained by alternately laminating piezoelectric layers made of, for example, a piezoelectric material and inner electrode layers having electrically conducting property, and having outer electrodes joined to the side surfaces of the ceramic laminated body. When a voltage is applied across the inner electrode layers, the piezoelectric layers undergo a displacement to drive the laminate-type piezoelectric element.
Some laminate-type piezoelectric elements have a non-pole structure in which inner electrode layers are not formed on the side surfaces, on the other side in the ceramic laminated body, to enhance electric insulation on the outer circumferential surface of the ceramic laminated body (see Japanese unexamined patent publication No. 2001-135872).
When the ceramic laminated is viewed in the direction of lamination, however, the laminate-type piezoelectric element of the non-pole structure has an overlapped portion which is a region where the inner electrode layers are all overlapped one upon the other and a non-overlapped portion which is a region where only some of the inner electrode layers are overlapped or none of them are overlapped. The non-overlapped portion does not undergo piezoelectric displacement and cannot be driven. Therefore, stress (strain) is concentrated in the non-overlapped portion depending upon the piezoelectric displacement, and cracks may occur in the ceramic laminated body. Also, there may occur an inconvenience such as an insulation breakdown.
In order to solve the above problem, Japanese unexamined patent publication No. 8-274381 and Japanese unexamined patent publication No. 2001-267646 propose structures, for laminate-type piezoelectric elements, provided with stress relaxing portions (stress relaxing layers) among the inner electrodes to relax the stress. However, the stress relaxing portions in the above patent publications are filled with, for example, fine lead titanate particles which may migrate upon being repetitively driven and may be detrimental to the performance of the piezoelectric element.
Japanese unexamined patent publication No. 2001-210886 proposes a structure of a laminate-type piezoelectric element in which recessed grooves are formed in the ends of the inner electrodes by machining and are filled with an insulator or a conductor to impart the function of relaxing the stress. According to this patent publication, however, the recessed grooves are formed by machining which is not suited for mass production. Besides, cracks develop on the machined surfaces which may become a cause of an inconvenience such as an insulation breakdown.
It is therefore desired to provide a method, of producing a laminate-type piezoelectric element, which is capable of forming a structure for relaxing the stress caused by piezoelectric displacement by a simple method and is, further, capable of providing excellent durability and reliability by relying upon the stress-relaxing structure.